The present invention relates to cryogenic processes and plant for separation of air with production of oxygen under high pressure.
Conventionally, the production of oxygen under high pressure, meaning for example about 40 bars in the present context, is performed by simple compression of oxygen in the gaseous state suppplied by the low-pressure downflow section of a cryogenic air separation plant comprising a medium-pressure upstream section and a low-pressure downstream section. This compression of oxygen in the gaseous state is troublesome and the compression equipment is delicate and possibly unsafe or dangerous.
It has also been proposed to produce oxygen at the outflow from the low-pressure downstream section in the form of a fraction in the liquid state under low pressure, which is pumped to the high pressure from the said low pressure in the liquid state and which is exposed to a complete vaporization by thermal exchange in counterflow with fluids, of which the one or first fluid is air under a high pressure of the order of the said high pressure as hereinabove referred to, and a part of which is expanded to the medium pressure aforesaid before being fed, in at least partly liquid form, into at least one separation section, and the other or second fluid of which is air under an intermediate pressure, and fed in the gaseous state into the said separation section.
Compared to the first process referred to above, this process offers the advantage of obviating the use of an oxygen compressor, but has the drawback of leading to a higher overall power consumption as soon as a high oxygen production pressure prevails. This second process cannot be acceptable in respect of energy demand unless the oxygen vapourisation temperature remains below that of the high-pressure air condensing in counterflow with the vapourisation of this oxygen. Because of this, as moderate an oxygen pressure as 15-20 bars requires an air pressure already reaching 50-60 bars. The oxygen pressure lies between 40 and 100 bars for many applications, so that the condition specified above can no longer be fulfilled, the equipment utilised and in particular the exchangers, not rendering it possible to raise the air pressure substantially above these pressure levels.
It is an object of the invention to provide a process which economically renders it possible to obtain oxygen under high pressure by compression of an oxygen fraction in the liquid state.